linux-kernel - Re: [tip:x86/urgent] x86/tls: Don't validate lm in set_thread

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Message-ID: <CALCETrU=+rBkG_xwaOA4vruiAeHFE_XLPrAgVCjnvpO5WrfgTw@mail.gmail.com>
Date:	Thu, 18 Dec 2014 10:26:57 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/urgent] x86/tls: Don't validate lm in set_thread_area()
 after all

On Thu, Dec 18, 2014 at 8:59 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On 12/18/2014 03:16 AM, tip-bot for Andy Lutomirski wrote:
>> Commit-ID:  3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
>> Gitweb:     http://git.kernel.org/tip/3fb2f4237bb452eb4e98f6a5dbd5a445b4fed9d0
>> Author:     Andy Lutomirski <luto@...capital.net>
>> AuthorDate: Wed, 17 Dec 2014 14:48:30 -0800
>> Committer:  Ingo Molnar <mingo@...nel.org>
>> CommitDate: Thu, 18 Dec 2014 12:12:26 +0100
>>
>> x86/tls: Don't validate lm in set_thread_area() after all
>>
>> It turns out that there's a lurking ABI issue.  GCC, when
>> compiling this in a 32-bit program:
>>
>> struct user_desc desc = {
>>       .entry_number    = idx,
>>       .base_addr       = base,
>>       .limit           = 0xfffff,
>>       .seg_32bit       = 1,
>>       .contents        = 0, /* Data, grow-up */
>>       .read_exec_only  = 0,
>>       .limit_in_pages  = 1,
>>       .seg_not_present = 0,
>>       .useable         = 0,
>> };
>>
>> will leave .lm uninitialized.  This means that anything in the
>> kernel that reads user_desc.lm for 32-bit tasks is unreliable.
>>
>
> No, it won't.  However, if you initialize this dynamically field by
> field rather than as an initializer, then you are correct.
>

I tried the code above in function scope.

--Andy

>         -hpa
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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